Hydrogenation catalyst based on a platinum metal partially poisoned with finely divided sulfur

ABSTRACT

A hydrogenation catalyst, obtainable by carrying out essentially the following steps: 
     (a) treatment of a platinum metal salt with finely divided sulfur followed by 
     (b) reduction of the platinum metal salt to metallic platinum metal.

The present invention relates to a hydrogenation catalyst which can beobtained by carrying out essentially the following steps:

(a) treatment of a platinum metal salt with finely divided sulfur,followed by

(b) reduction of the platinum metal salt to metallic platinum metal.

The invention also relates to a process for the preparation of thehydrogenation catalyst of the invention, to its use as hydrogenationcatalyst particularly for the manufacture of hydroxylammonium salts, andto a process for the regeneration of hydrogenation catalysts.

The expense incurred in the preparation and regeneration of a catalystand the activity, selectivity, and useful life of the catalyst usedconstitute a decisive factor in determining whether a catalyzed reactionis feasible for industrial use or not. This is the case, for example,when effecting catalytic reduction of nitrogen monoxide with hydrogen toform hydroxylammonium salts. Hydrogenation catalysts containing platinummetal are normally used in this process.

Processes for the preparation of hydroxylammonium salts are known, forexample, from DE-PS 956,038 and DE-A 4,022,851. The catalysts used insaid references are obtained by precipitation of platinum on tosuspended graphite supports if desired with the addition of poisoningmeans such as compounds of sulfur, selenium, arsenic, or tellurium. Thedrawbacks of these processes are, on the one hand, the unduly rapid lossof activity and selectivity, and on the other hand, the use of anexpensive manufacturing process having a deleterious effect on theenvironment.

Thus it has hitherto been usual to buffer the reaction mixturecontaining a platinum metal salt with, say, sodium acetate prior to theaddition of the catalyst poison. The buffer is contaminated with organicsubstances after the reaction has taken place and this must be removedfrom the waste water, which incurs high disposal costs. Furthermore,buffering is expensive, particularly when use is made of sodium thioniteas donor for the sulfur acting as catalyst poison, due to the fact thatit is necessary to set up a redox potential.

The sodium thionite mentioned in DE-PS 956,038 is also unstable inatmospheric oxygen, and it must be handled in compliance with certainsafety precautions (avoidance of heating above 50° C. and the occurrenceof flying sparks, storage in a cool and dry atmosphere).

Other sulfur donors as yet proposed for partial poisoning of thecatalyst have equally decisive drawbacks: thiourea is considered to becarcinogenic, sodium thiosulfate and sodium sulfite are decomposable,sodium sulfide is considered to be toxic and moreover, these poisoningagents are not conducive to satisfactory catalyst properties.

It is thus an object of the present invention to provide hydrogenationcatalysts which do not exhibit the aforementioned drawbacks. Inparticular, it is desirable to simplify the preparation of hydrogenationcatalysts, to reduce the amounts of waste produced, and to avoid the useof readily decomposable and toxic substances for partial poisoning.

Accordingly, we have found the hydrogenation catalysts defined above.

We have also found a process for the preparation and regeneration of thehydrogenation catalysts of the invention as well as a method of usingthem for the preparation of hydroxylammonium salts.

The hydrogenation catalysts of the invention are obtained by thetreatment of a platinum metal salt with finely divided sulfur andsubsequent reduction of the platinum metal salt thus treated to metallicplatinum metal.

Theoretically, all platinum metal salts which are suitable forhydrogenation can be used as platinum metal salts, eg, salts of nickel,palladium, platinum, cobalt, rhodium, iridium, and ruthenium andpreferably palladium and platinum and more preferably platinum. Thewater-soluble salts of these metals are especially suitable, such as, inparticular, the halides, nitrates and sulfates. The following areexamples thereof:

platinum(IV) compounds such as hexachloroplatinic acid and alkali metaland ammonium salts thereof, tetrachloroplatinate ortetrachlorodihydroxyplatinic acid;

platinum(II) compounds such as tetrachloroplatinic acid and alkali metalsalts thereof or platinum(II) chloride;

palladium(II) compounds such as hexachloropalladous acid and saltsthereof or palladium(II) chloride.

Theoretically, mixtures of substantially platinum metal or platinummetal salts with other metal salts, for example salts of arsenic,antimony, selenium or tellurium can be used.

In accordance with the present invention finely divided sulfur is usedas sulfur serving to effect partial poisoning, for example commercial"flowers of sulfur". Sulfur having a particle size smaller than 500 μmand preferably smaller than 50 μm is preferably used, and morepreferably use is made of a sulfur having a particle size distributionin which 20% of the particles are smaller than 1 μm, 50% of theparticles are smaller than 5 μm and 90% of the particles are smallerthan 10 μm. Suitable sulfur is commercially available, for example aswettable sulfur "Humulus WG" (BASF) or can be obtained from, forexample, flowers of sulfur or finely ground sulfur by conventionalmethods, in particular by sifting.

The platinum metal salt is usually treated with the finely dividedsulfur in aqueous solution by contacting the aqueous platinum metal saltsolution with the finely divided sulfur. The sulfur can also be used ascolloidal sulfur solution (cf Jander-Blasius, Einfuehrung in dasanorganisch-chemische Praktikum, 5th Edition, 1964, pp. 415). It ispreferred to add the sulfur in the form of an aqueous suspension.

Theoretically, it is possible to use other solvents instead of, or inaddition to, the preferred solvent water.

Substances which improve the solubility or dispersibility of thestarting compounds can be added to the reaction mixture. In particular,as far as has been observed, all commonly used surfactants are suitablefor this purpose, in order to improve the solubility and wettability ofthe sulfur.

Suitable surfactants, which are also called dispersing agents, aredescribed, for example, in Ullmanns Encyklopaedie der technischenChemie, 4th Edition, Vol. 23, Verlag Chemie, Weinheim, 1983, pp. 31-39.The following are examples thereof:

Polyacrylates, poly(vinyl sulfonate)s, polyvinylpyrrolidone, TAMOL®(BASF), Schaeffer's salt as well as lignin sulfonates.

In a particularly preferred embodiment, lignin sulfonates are used assurfactant (known per se, eg, from UIImann, Encyclopaedie der techn.Chemie, 4th Edition, Vol. 16, pp. 253 et seq, Verlag Chemie, 1978) andpreferably alkali metal lignin sulfonates such as sodium and potassiumlignin sulfonates, since they can be readily removed with the washingwater when the finished catalyst is washed, and they have no adverseeffect on the environment on account of their good degradabilityproperties.

The surfactants are generally added to the reaction mixture prior to theaddition of the sulfur to the platinum metal salt or they areadvantageously added to the aqueous sulfur suspension.

The ratio, by weight, of surfactant to sulfur is usually adjusted so asto be in the range of from 0.1 to 50 and preferably from 1 to 15 wt %.As far as we have observed, more than 50 wt % of surfactant provides nosignificant improvement in the solubility of the sulfur, and, generally,no detectable improvement is achieved when use is made of amounts below0.1 wt %.

The temperature during the treatment of the platinum metal salt with thefinely divided sulfur is normally adjusted so as to be in the range offrom 20° to 95° C. and preferably from 40° to 95° C. and more preferablyfrom 50° to 85° C.

The pH during the treatment of the platinum metal salt with the finelydivided sulfur is usually adjusted so as to be in the range of from 1.5to 11.5 and preferably from 2.5 to 8.5, more preferably from 4.5 to 8.5and most preferably from 7.0 to 7.5.

The duration of the treatment of the platinum metal salt with the finelydivided sulfur, ie the time extending from the addition of the finelydivided sulfur to the addition of the reducing agent, is usuallyadjusted so as to be in the range of from 0.5 to 60 min and preferablyfrom 2 to 15 min. Treatment for less than 0.5 min generally providesinadequate poisoning of the catalyst, treatment for more than 60 minhas, as far as has been observed, no further advantage.

The molar ratio of platinum metal to sulfur is usually adjusted so as tobe in the range of from 90 to 10 and preferably of 75 to 35%.

Following partial poisoning with sulfur, the platinum metal salt isreduced to metallic platinum metal by adding, advantageously, a reducingagent to the reaction mixture obtained following the treatment of theplatinum metal salt with finely divided sulfur.

Normally all prior reducing agents capable of reducing platinum metalsalts to platinum metal are suitable, eg, hydrazine, formaldehyde,formic acid, or an alkali metal or alkaline-earth metal formate such assodium, potassium, and calcium formates and more preferably formic acid.

The molar ratio of platinum metal to reducing agent is usually adjustedso as to be in the range of from 0.5 to 100 mol % and preferably from 5to 85 mol %.

The temperature during the reduction is normally adjusted so as to be inthe range of from 20° to 95° C. and preferably from 40° to 95° C. andmore preferably from 50° to 85° C.

The pH used for the reduction substantially depends on the amount ofreducing agent used and the nature thereof. For example, when formicacid is used, the pH is usually adjusted to a value between 0.5 and 3.5and preferably between 1.0 and 2.5.

On completion of the reduction, the catalyst is usually worked up inconventional manner, for example by removing it from the reactionmixture by filtration, and washing it, advantageously with water.

In a preferred embodiment, the reduction and, if desired, the treatmentwith finely divided sulfur is carried out in the presence of a catalystsupport such as graphite or activated charcoal, preferably graphite. Itis especially preferred to admix the platinum metal salt with finelydivided graphite prior to the treatment with finely divided sulfur, thegraphite usually having a particle size ranging from 0.1 to 1000 μm andpreferably of from 0.1 to 300 μm and more preferably from 5 to 100 μm.The molar ratio of carbon (ie graphite or activated charcoal) toplatinum metal is generally adjusted so as to be in the range of from99.99 to 10 mol % and is preferably from 99.99 to 30 mol %. Inparticular, when platinum is the metal concerned, the said molar ratiois preferably from 99.99 to 90 mol % and more preferably from 99.98 to95.0 mol %.

The catalysts obtained using the process of the invention are suitablefor the hydrogenation of both organic and inorganic compounds, as far ashitherto discerned.

It is preferred to use the catalysts of the invention for thehydrogenation of olefinically or acetylenically unsaturated compounds,and also for the hydrogenation of carboxylic acids, aldehydes, orketones to the corresponding alcohols or of nitriles to thecorresponding amines. The catalysts of the invention are also suitablefor the hydrogenation of inorganic substances such as oxygen, but areparticularly useful for the preparation of hydroxylammonium salts by thehydrogenation of nitrogen monoxide in aqueous mineral acids.

In the manufacture of hydroxylammonium salts, a molar ratio of hydrogento nitrogen monoxide of from 1.5.:1 to 6:1 and preferably from 3:1 to5:1 is usually mantained. Particularly good results have been found tobe obtained when care is taken to ensure that a molar ratio of hydrogento nitrogen monoxide of from 3.5:1 to 5:1 is maintained in the reactionzone.

The acid used is advantageously a strong mineral acid such as nitricacid, sulfuric acid, or phosphoric acid. Acid salts such as ammoniumbisulfate are generally also suitable. It is usual to use from 4N to 6Naqueous acids, and the acid concentration is not usually allowed to fallbelow 0.2N during the course of the reaction.

The hydrogenation of nitrogen monoxide is generally carried out at atemperature in the range of from 30° to 80° C. and preferably from 35°to 60° C. Also, the pressure during the hydrogenation is generallyadjusted so as to be in the range of from 1 to 30 and preferably from1.5 to 20 bar (absolute).

The ratio of mineral acid to catalyst depends on the platinum metalused, and is, in the case of platinum/graphite catalysts, generally inthe range of from 10 to 100 g and preferably from 30 to 80 g of platinumper liter of mineral acid.

In a further preferred embodiment, in particular in the manufacture ofhydroxylammonium salts, the catalyst is treated ("activated"), prior tothe actual hydrogenation, with hydrogen in acid solution, advantageouslyin the mineral acid in which the hydrogenation is to be carried out.

Spent platinum metal catalysts can be regenerated with the aid of theprocess of the invention by dissolving the platinum metal of thecatalyst normally by means of an acid or an acid mixture, and separatingany insoluble constituents. The platinum metal salt solution obtained isthen neutralized, and the platinum metal salt is then treated accordingto the process of the invention described above by mixing it with finelydivided sulfur and then reducing the platinum metal salt thus treatedwith a reducing agent, whilst a support material can be added prior to,during, or subsequently to the treatment with finely divided sulfur ifdesired.

As far as we have observed, the catalysts of the invention are superiorto prior art catalysts used for the the same purpose as regardsactivity, selectivity and maximum on-stream time. The process of theinvention for the preparation of and regeneration of hydrogenationcatalysts has the advantage over prior processes in that it is simplerto carry out and at the same time reduces the amount of waste productsproduced and avoids the use of readily decomposable and toxic substancesfor partial poisoning.

EXAMPLES Hydrogenation Catalyst Based on a Platinum Metal PartiallyPoisoned with Finely Divided Sulfur

The particle size was determined using a Malvern Mastersizer (cfVerfahrenstechnik 24 (1990) pp. 36 et seq) by measuring the Frauenhoferdiffraction at a wavelength of 633 nm. The use of a supplementary lenshaving a focal length of 300 mm made it possible to determine theparticle size distribution over a range of from 1 to 600 μm.

Measurement was effected by adding a spatula-tipful of the test powderto one liter of a 0.1 wt % strength aqueous solution of Nekanil® 910(BASF AG, Nekanil® 910 is a nonyl phenol which has been caused to reactwith from 9 to 10 mol of ethylene oxide; properties: water-white,viscous liquid; non-ionic; density at 20° C. 1.04 g/cm³ ; pour-point:below -10° C.; pH of a 1 wt % strength solution: 6.5 to 8.5). Theresulting test mixture was subjected to ultrasonic treatment for oneminute before readings were taken.

EXAMPLE 1

640 g (calculated as dry substance) of a finely divided graphite support(particle size in the range of from 0.1 to 300 μm) were suspended in 500mL of water and 100 mL of aqua regia, 0.5 wt % of platinum was added ashexachloroplatinic acid hexahydrate, and the mixture was stirredovernight at a temperature of 80° C. The next day the suspension wasdiluted with 400 mL of water, cooled to 30° C., and adjusted to pH 7.5with soda. 150 mg of finely divided elementary sulfur suspended in 50 mLof water (distribution of particle sizes: 20%<1 μm; 50%<5μm; 10%<10 μm)were added to the reaction mixture. Following a period of 10 min, 100 mLof 99 wt % strength formic acid were then added to the reaction mixture.

50 g of the catalyst thus treated were suspended in 1250 mL of 20 wt %strength H₂ SO₄, and treated with H₂ at a temperature of 40° C.

121L/h of a mixture of 67.7 vol % of H₂ (99.95%strength) and 32.3 vol %of NO (99.6% strength) were then fed to the suspension.

Following the introduction of a total of 423 L of the above gas mixturethere were obtained 1342 mL of a solution having the followingcomposition:

18.5 g/L H₂ SO₄

108.1 g/L NH₂ OH (as (NH₃ OH)₂ SO₄)

7.3 g/L NH₃ (as (NH₄)₂ SO₄)

The total amount of off-gas was 91.6 L of the following composition:

76.7 vol % of H₂

19.5 vol % of NO

2.8 vol % of N₂ O

The following values for selectivity and space-time yield can becalculated from these results:

Selectivity:

85.2% toward NH₂ OH

10.1% toward NH₃

4.7% toward N₂ O

Space-time yield:

1.12 mol/L_(RR) fl.·h, based on NO conversion (consumption)

0.96 mol/L_(RR) fl.·h, based on NH₂ OH, formed

where mol/L_(RR) fl..h denotes moles per liter of fluid reaction spaceper hour.

EXAMPLE 2

A suspension of 640 g (calculated as dry substance) of a finely dividedgraphite support (particle size in the range of 0.1 to 300 μm), 500 mLof water, 100 mL of aqua regia, and 0.5wt % of platinum in the form ofhexachloroplatinic acid hexahydrate was stirred overnight at atemperature of 80° C. The next day, the suspension was diluted with 400mL of water, cooled to 30° C., and adjusted to pH 7.5 using soda. Asuspension of 150 mg of finely divided elementary sulfur (distributionof particle sizes: 20%<1 μm; 50%<5 μm; 90%<10 μm), 20 mg of sodiumlignin sulfonate, and 50 mL of water was then added. Following a periodof 10 min, 100 mL of 99 wt % strength formic acid were added to thereaction mixture.

50g of the catalyst thus treated were suspended in 1250mL of 20 wt %strength H₂ SO₄ and treated with H₂ at a temperature of 40° C.

121 L/h of a mixture of 68.0vol % of H₂ (99,95% strength) and 32.0 vol %of NO (99.6% strength) were then fed to the suspension.

Following the introduction of a total of 406 L of the above gas mixturethere were obtained 1340 mL of a solution having the followingcomposition:

17.2 g/L H₂ SO₄

193.5 g/L NH₂ OH (as (NH₃ OH)₂ SO₄)

13.3 g/L NH₃ (as (NH₄)₂ SO₄)

The total amount of off-gas was 77.7 L of the following composition:

76.1 vol % of H₂

19.6 vol % of NO

3.0 vol % of N₂ O

The following values for selectivity and space-time yield can becalculated from these results:

Selectivity:

76.1% toward NH₂ OH

19.5 % toward NH₃

4.4 % toward N₂ O

Space-time yield:

1.1 8 mol/L_(RR) fl..h, based on NO conversion (consumption)

0.90 mol/L_(RR) fl..h, based on NH₂ OH.

COMPARATIVE EXAMPLE

A suspension formed from 640 g (calculated as dry substance) of a finelydivided graphite support (particle size in the range of from 0.1 to 300μm), 500 mL of water, 100 mL of aqua regia, and 0.5 wt % of platinum ashexachloroplatinic acid hexahydrate was stirred overnight at atemperature of 80° C. The next day, the suspension was diluted with 400mL of water, cooled to 30° C., and adjusted to pH 3.0 with soda. 80mL ofa 40 wt % strength sodium acetate solution was then passed, as buffersubstance, into the reaction mixture. 15 mL of a 5 wt % strength Na₂ S₂O₄ solution were then added until a redox potential of 430 mV wasachieved. mL of 99 wt % strength formic acid were then added to thereaction mixture. A Pt/Ag/AgCl electrode (Metrohm, Herisau) served asredox electrode. Calibration was effected using a redox calibratingsolution for a Pt/Ag/AgCl electrode (Schott-Geraete GmbH, Hofheim)having a redox potential of +475 mV at a temperature of 20° C.

50 g of the catalyst thus treated were suspended in 1250 mL of 20 wt %strength H₂ SO₄ and treated with H₂ at a temperature of 40° C.

100 L/h of a mixture of 70.0 vol % of H₂ (99.95% strength) and 30.0 vol% of NO (99.6% strength) were then fed to the suspension.

Following the introduction of a total of 300 L of the above gas mixturethere were obtained 1338 mL of a solution having the followingcomposition:

84.7 g/L H₂ SO₄

55.5 g/L NH₂ OH (as (NH₃ OH)₂ SO₄)

13.2 g/L NH₃ (as (NH₄)₂ SO₄)

The total amount of off-gas was 59.0 L of the following composition:

77.0 vol % of H₂

15.4 vol % of NO

8.6 vol % of N.sub. O

The following values for selectivity and space-time yield can becalculated from these results:

Selectivity:

60.0% toward NH₂ OH

27.8% toward NH₃

12.2% toward N₂ O

Space-time yield:

0.99 mol/L_(RR) fl..h, based on NO conversion (consumption)

0.59 mol/L_(RR) fl..h, based on NH₂ OH formed.

We claim:
 1. A process for the preparation of a hydrogenation catalystconsisting essentially of(a) treating a platinum metal salt with finelydivided sulfur, and (b) reducing the treated platinum metal salt tometallic platinum metal.
 2. A process as defined in claim 1, wherein theparticle size of the finely divided sulfur is less than 500 μm.
 3. Aprocess as defined in claim 1, wherein the reduction (step (b)) iscarried out in the presence of a catalyst support material.
 4. A processas defined in claim 1, wherein the process steps are carried out in anaqueous medium.
 5. A process as defined in claim 1, wherein thetreatment with finely divided sulfur is carried out in the presence of asurfactant.
 6. A process as defined in claim 1, wherein the molar ratioof platinum metal to sulfur is in the range of from 90 to 10.